A weather radar apparatus using a parabolic antenna transmits a pulse signal by a thin beam named a pencil beam in a procedure according to a preset observation sequence. The observation sequence shows an observation procedure when the weather radar apparatus observes surroundings. The weather radar apparatus collects three-dimensional rain data by repeating a procedure in which an antenna is rotated horizontally by 360 degrees at a predetermined elevation angle to obtain observational data in one plane and data in a next plane is then obtained with the elevation angle increased.
On the other side, spatial distribution of a target rain differs for each phenomenon and varies every moment. In order to minimize damage inflicted by microburst, a shear line, a local severe rain, and lightning which a cumulonimbus cloud accompanies, it is effective to check a growth degree of a cumulonimbus cloud in a perpendicular direction. However, with the weather radar apparatus described above, an antenna is scanned according to the preset observation sequence, and therefore, only conditions of the cumulonimbus cloud can be observed at the preset elevation angle. A center part of a cumulonimbus cloud cannot be observed with high density. Therefore, estimation of the growth degree of a cumulonimbus cloud in a perpendicular direction can include an error.